Wheel slip correction systems commonly used in locomotive applications in DC electric traction motors are coupled in driving relationship to wheel/axle assemblies of the locomotive. Because the locomotive wheels are steel running on steel rails there is a high likelihood of one or more wheels slipping either from effects of load or rail conditions. Various methods have been used to overcome wheel slip and improve traction including mechanical methods, such as applying sand to a rail, and electrical methods such as by removing or decreasing electrical power to a motor driving the slipping wheel.
One method of electrically correcting for wheel slip is disclosed in U.S. Pat. No. 6,634,303. In this patent, a plurality of DC electric motors connected for driving respective wheels of a locomotive have their field windings shunted by a resistance so that at least some armature current bypasses the field winding so that the motors are normally operated in a field weakened mode. Upon detection of a wheel slip, a switch that is connected in series circuit with the resistance is opened so that full armature current is forced through the field winding of the motor associated with the slipping wheel. The increase in field current increases motor flux which in turn increase motor back EMF to thereby reduce the effective voltage at the motor armature causing a drop in motor current and a concomitant reduction in motor torque. Reduced torque allows the wheel to slow thereby correcting for the wheel slip. An obvious disadvantage of this system is the power loss in all the resistive loads connected in parallel with the field windings of each of the motors during normal operation, particularly since wheel slipping is a random, short-term event.